And it is very slow even in web-browsing on clean install SSD!
What are your ideas what to upgrade?

I'm sure that I need to upgrade it to at least 12-24 or better 48GB of RAM (As it is problem occures with 64 GB on these Macs!!!). Ok, it would cost about 200$.

Then, I want to upgrade disk subsystem into PCIe SSD adapter (it is an adapter to use M2 SSDs x2 in RAID and 4 to 6 external SATA-III ports). As I think that cMacPro bottle neck is SATA-II interface. And I hope that it would solve all the problems with freezes and slow work (combined with upgraded RAM). Am I right? This adapter costs $100 + 2 M2 SSDs - 100+$ each. I'm not sure in need of M2 RAID configuration, so t first time I will use my existing Crucial M2 128GB and Crucial M2 256GB SSDs and spend only $100 for adapter for SATA ports. Most important, I want to use these drives as bootable! (As I understand any PCIe -> Sata III or M2 adapter could be used, just use AHCI SSD drives and it is bootable). I also decided not to Buy any OWC Accelsior or Sonnet Tempo adapters as they very expensive and has no additional SATA-III ports for other drives.

I already tried to upgrade VideoCard into ATI Radeon 7950 (flashed for Mac and has bootscreen). I have good result in FCPX but not much noticeable (i think because of SATA-II drives), also I have Nvidia GTX 760 2GB card for my experiments (I can't turn it on my system as it has 6+4 power cables, mac has 6+6 and I little bit aware of using adapters for 6-to-8 pin video cable) . Also, I saw no big difference with GTX 285. There are no difference in encoding time at all, as I don't use effects at the moment.

To buy 2 x E5472 6-core 3.0Ghz CPUs. I make a decision to 3.0 Ghz instead of 3.2 (max upgrade for this motherboard) as there are very little difference cannot be seen in speed, but TDP is much more on 3.2 (approx. 85W vs 120W) and it could take more money on electricity for nothing. Also, I decided to buy E5472, not X5472 as there are no any difference instead of bigger TDP and cost in X5472 version. Right now my Adobe Media Encoder uses all processor cores at 100%, so I think work will be done faster with 2 processors and more cores? I think used processors could cost me about 120$ + additional Apple cooler for the second CPU.

Also I'm considering of buying OWC Dual Pro RAID External Case (4TB of 6TB RAID with Thunderbolt and USB3) + USB 3.0 or USB 4.1 PCIe adapter for Mac Pro. It gives about 400MB/S reading and writing over USB or Thunderbolt (I have MacBook Pro with Thunderbolt display, by the way). And I think this speed is enough for video editing? Also, I would have fast external storage (It even could be bootable via Thunderbolt or USB, as I understand).

I added numerals to the steps as I interested in discussing each of them separately.

After all upgrades I would spend 200 + 100 + 120 + 60 (let it be for Apple CPU cooler used + shipping) = 580$.

Will I get after all upgrades an excellent machine for editing Full HD (Concerts, Weddings in the future after I get more experience) and fast export of SD (720 x 576) videos (as I working a lot with VHS tapes) and working with photos in Photoshop/Lightroom?

1) more RAM sure will help. In my experience, the RAM really required under heavy duty are usually stay at ~24GB (I have 48GB RAM, as per my signature). The extra RAM will work as cache. But not really that necessary. And that 24GB consumption including 2 users profiles constantly logged in, few browsers always opened, and some media server activity in the background.

2) SATA II is bottlenecking the SSD and may eventually bottleneck the system. However, it really depends on the job. e.g. It will bottleneck the system when loading a very large video, but it won't be the bottleneck during encoding.

3) there is a power management bug for HD7950 on 3,1, you better check it out. It affect the 7950's performance a lot. In any case, encoding time basically won't affected by GPU performance. That's CPU's job.

4) yes, Adobe ME can utilise lots of cores, an extra CPU basically means can finish the same job in half time. Faster CPU will also help.

5) not sure will your workflow benefit by that HD enclosure. I can edit 4K video on my 4,1 with just a 840 Evo plugged into one of the SATA II port. I have the SATA III card, I tried that, but that doesn't give me much benefit. So I remove the card and install a 2nd 7950, which is more useful for my video editing work. For SD video editing, SATA II bandwidth should be more than enough.

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